Conditions for achieving efficient ionization of a molecule to be analyzed in a MALDI (Matrix-Assisted Laser Desorption/Ionization) mass spectrometry method have been searched. For example, JP-A-2005-326391 (Patent Document 1) discloses a method in which a hydrophobic peptide is more efficiently ionized in mass spectrometry by previously modifying the hydrophobic peptide with a 2-nitrobenzenesulfenyl group and using, as a matrix, α-cyano-3-hydroxycinnamic acid (3-CHCA), 3-hydroxy-4-nitrobenzoic acid (3H4NBA), or a mixture of them than by using a common matrix such as α-cyano-4-hydroxycinnamic acid (4-CHCA) or 2,5-dihydrobenzoic acid (DHB).